It is generally known that urethane prepolymers can be prepared by reaction of a polyisocyanate with an intermediate. The intermediate is a relatively high molecular weight compound and is usually terminated with hydroxyl functions. The polyisocyanate is usually a diisocyanate. Thus the reaction of the polyisocyanate with the intermediate polyol produces a urethane prepolymer with functional groups on each end of the prepolymer. If a diisocyanate is reacted with a diol in a molar ratio of one isocyanate to one hydroxyl function, theoretically a single polymer molecule would be formed having an isocyanate function on one end and a hydroxyl function on the other end. Under practical conditions, a single prepolymer molecule does not form because of various side reactions. The product is generally a very viscous fluid or solid which is difficult to handle and, therefore, of little practical utility. For this reason, urethane prepolymers are generally formed by reacting a polyol with an excess of polyisocyanate so that each prepolymer molecule is terminated with an isocyanate function. Generally, the molar ratio of isocyanate to hydroxyl is at least 2:1 so that a single diol will react with two diisocyanate molecules to form a prepolymer composed of a single diol molecule with a molecule of the diisocyanate reacted on each end. This prepolymer is then chain extended, usually with a low molecular weight diol and usually using a hydroxyl isocyanate molar ratio of 1:1, to form a polyurethane of high molecular weight which can be molded or cast into useful products.
In the field of our invention, a large excess of polyisocyanate is used so that the "hard segments" which are the urethane functions formed by the reaction of the prepolymer isocyanate functions as well as the excess diisocyanate with the chain extender (usually a low molecular weight diol) can be increased to the order of 25% to 50% by weight of the final polyurethane.
The preparation of prepolymers from diisocyanates and diols having the same repeating unit but different molecular weights and the preparation of prepolymers having different repeating units but essentially the same or different molecular weights can be prepared in a manner as set forth above.
According to the present invention, prepolymers are prepared by blending at least two different intermediates and subsequently reacting the blend with a polyisocyanate. Although one skilled in the art would expect that similar prepolymers are formed, it has been unexpectedly determined that the method of the present invention produces a urethane prepolymer which, when chain extended, produces a polyurethane with physical properties generally superior to polyurethanes prepared by the prior art method.